Preheat the oven to 425 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, toss the squash with 2 tablespoons of the olive oil and sprinkle with salt and pepper. Spread in a single layer on the prepared baking sheet and roast until golden and caramelized at the edges, stirring at least once, 40 to 55 minutes. Let cool.
Bring the vegetable stock to a boil in a small saucepan. Reduce the heat and maintain at a bare simmer.
Heat the remaining 4 tablespoons oil in a large, high-sided, heavy-bottomed skillet over medium heat. Add the shallots and cook until soft, 3 to 4 minutes. Add the garlic; cook just until it releases its fragrance, about 30 seconds, then add the rice and stir to coat. Sprinkle with salt and pepper. Stir in the wine and cook until almost completely evaporated. Reduce the heat to medium-low and add a ladleful of stock; cook, stirring constantly, until almost completely absorbed. Add another ladleful; again, cook, stirring, until almost fully absorbed. Continue in this manner until the rice is creamy and al dente (tender but still firm to the bite), about 40 minutes--you may or may not use all the stock.
Remove the risotto from the heat and stir in the Parmesan if using and the squash, reserving some cubes for garnish. Add salt and pepper to taste. Serve immediately, garnished with the reserved squash cubes.